As a simple and very effective method to suppress noise in a sound signal on which noise is superimposed, spectral subtraction that is proposed by S. F. Boll is known. By this spectral subtraction, gain is calculated using a power spectrum of a noise-superimposed sound of a current frame (for example, Non-Patent Literature 1).
Moreover, there is a method of calculating gain using a power spectrum of a noise-superimposed sound on which time-direction smoothing is performed. According to this method, to reduce the effect of a cross-correlation term, power spectrums of noise-superimposed sound of a current frame and some past frames are moving-averaged in a time direction to be smoothed. In other words, gain is calculated using a power spectrum of a time-direction-smoothed noise-superimposed sound on which time-direction smoothing is performed (for example, Non-Patent Literature 2).    Non-Patent Literature 1: S. F. Boll “Suppression of Acoustic Noise in Speech Using Spectral Subtraction”, IEEE Transaction on Acoustics, Speech and Signal Processing, 1979, ASSP Magazine Vol. 27, No. 2, pp. 113-120    Non-Patent Literature 2: Norihide Kitaoka, Ichiro Akahori, and Seiichi Nakagawa “Speech Recognition Under Noisy Environment Using Spectral Subtraction and Smoothing in Time Direction”, The Institute of Electronics, Information and Communications Engineers, February 2000, Vol. J83-D-II, No. 2, pp. 500-508